MYSQL_HISTFILE and .mysql_history

The MySQL manual says:
"mysql Logging
On Unix, the mysql client logs statements executed interactively to a history file. By default, this file is named .mysql_history in your home directory. To specify a different file, set the value of the MYSQL_HISTFILE environment variable."
The trouble with that is: it doesn't tell you what you don't need to know. So I'll tell you.

Heritage

The history-file concept that MySQL and MariaDB are following is indeed "on Unix" and specifically is like the GNU History Library. There is a dependence on external libraries, Readline or EditLine/libedit, depending on the licence. The similarity to (say) the Bash shell history is clear when one compares some names and some actions.

Thing

Bash

MySQL

Default file name

$HOME/.bash_history

$HOME/.mysql_history

Environment variable for name

HISTFILE

MYSQL_HISTFILE

Environment variable for ignoring

HISTIGNORE

MYSQL_HISTIGNORE

This explains why the history file is hidden, why the facility doesn't work on Windows, and why there's no orthogonality (that is, one can set an environment variable but one cannot set a configuration-file or command-line parameter).

Is it really a log?

The manual says .mysql_history is a log, but that's a bit misleading, since it's not much like the "logs" that the server produces (the transaction log, the slow query log, etc.). There are no timestamps; there is a large class of SQL statements that are ignored such as ones that come via file input; there is no way to know which statements succeeded and which ones ended with errors.

So the only thing .mysql_history is truly good for is making some edits easier. If you like to say "repeat last statements", or some sophisticated variant that requires knowing what earlier statements were, then the mysql client -- actually, the Readline or EditLine library -- needs the history for that.

Also it's occasionally useful for answering questions like "How did I get myself into this mess?" when odd things start to happen, and especially so when you have to write a bug report and your friendly MySQL or SkySQL or Percona support worker enquires: "Could you please show us the things you were doing just before the, er, anomaly occurred." A tee file would be better, but sometimes a histfile might do.

Format

Not all versions of the mysql client will write the .mysql_history file in the same way. That is why sometimes you will see "X Y" if you print the file, but sometimes you will see "X\040Y". Of course 040 is the octal for the ASCII for the space character, but this has led to bug reports such as Bug#14634386 - "History written by Libedit is not readable by Readline" and Bug#16633124 - Compatibility issue with mysql history ("\040" instead of space). These are Oracle internal bug numbers, so don't bother trying to find them on bugs.mysql.com. They're unimportant bugs, since they should not cause problems for editing.

A more severe matter is that multi-line statements are logged twice. First there is a record of each line. Then there is a record of the statement as a whole.

Stopping

Although disk space is cheap, people do worry sometimes when they realize their computer might be writing secret stuff in a file that burglars could see. The easiest prevention is to start the mysql client with --batch or an option that implies --batch. However, those options all do something else, as well as stopping writing to .mysql_history, so they're not "well targeted" solutions.

There's a recommendation to set the MYSQL_HISTFILE environment variable to say "/dev/null", or to link the .mysql_history file to /dev/null. Loosely one might think: oh, statements are being sent to /dev/null and therefore they won't get saved. More accurately, they might not be sent anywhere at all, as one can see from these snippets of mysql.cc code (Copyright (c) 2000, 2014, Oracle and/or its affiliates):

Limiting

Despite the similarities to Bash, there is no MYSQL_HISTSIZE environment variable to be like Bash's HISTSIZE. With some unreliable fiddling, one can try to tell the underlying library what the maximum history size should be, that is, how many lines are allowed in the history file before starting to discard. For example, instead of saying "/usr/bin/mysql", say

rlwrap -a --histsize=5 /usr/bin/mysql

Usually this particular trick is best avoided because of its side effects.

MYSQL_HISTIGNORE

In MySQL's mysql, there's a newish environment variable:

export MYSQL_HISTIGNORE=pattern

It's not in my copy of MariaDB's mysql, but I suppose we can expect it soon. The idea is: if a statement matches the pattern, then it doesn't get saved in the history file. But there are some flaws ...

There are only two wildcards for the pattern: "?" meaning "one occurrence of any character", and "*" meaning "zero or more occurrences of any character". There is no regular-expression variation, and the choice is un-SQL-ish (in SQL one would use _ and % not ? and *).

A certain pattern is always in effect so that passwords won't be saved. A fine default, but it's regrettable that there's no way to remove it.

It appears to be impossible to escape certain characters. For example, if I wanted the pattern to include a colon, I couldn't say "export MYSQL_HISTIGNORE=a\:b".

ocelotgui

The objective of Ocelot Computer Services Inc. is that the ocelotgui program will do anything that the mysql client can do, and one can dream that there are ways to do even more (porting to Windows, recognizing history size, matching more expressions, etc.) along with the advantages that a GUI automatically has over a command-line editor. But at the moment we don't have a histfile, we only support tee. I think that's the last big feature that ocelotgui needs before we call it 'feature complete', which is why I've been looking at .mysql_history details.